The Third Dimension
While a 2 or 3 space slipstream move may simply skip past an enemy unit on a flat level, a 1 space warp can go over or under an enemy unit with the next warp returning to the original level on the far side. A route requiring both vertical and horizontal movement is easier to block.
Lurkers In The Dark
Dark ships are useful as sacrifice popups, giving garrison fleet info including new builds and preventing movement by pinning those units in place for combat.
With Friends Like These
Your ally might be telling someone else where your fleets are located. A single ship at a planet will pin a fleet - including any new builds at that planet - in place for a turn if an ally suddenly becomes an enemy.